The heterogeneity of the aluminum structure and mechanical properties under equal-channel angular pressing

An analysis of the diffraction patterns from backscattered electrons and of measured microhardness has been performed to investigate the distributions of structural parameters (average grain/subgrain size and fraction of high-angle grain boundaries) and of the mechanical properties in the longitudinal section of a pure aluminum billet subjected to severe plastic deformation by equal-channel angular pressing. The distributions of the characteristics under study were found to be nonuniform. A minimum average grain size and a maximum fraction of high-angle boundaries were observed in the core of the billet. As the sample surface was approached, the examined parameters were seen to vary in magnitude through a broad range. Conceivable reasons for the structural heterogeneity of the material are discussed.     

Structural Changes in Iron upon Large Plastic Deformations and Their Influence on the Complex of Its Mechanical Properties

The influence of the degree of preliminary strain, obtained by equal-channel angular pressing, on the character of work hardening of highly deformed armco-iron is investigated. The effect of the deformation on the evolution of dislocation and cellular structures is analyzed. The structural sensitivity of the mechanical properties is studied.     

Mechanisms of high-speed deformation of polycrystalline copper

The mechanisms of high-speed deformation of ultrafine-grained copper produced during severe plastic deformation by equal-channel angular pressing were analyzed using numerical modeling in comparison with those in the case of coarse-crystalline copper. The activity of annihilation processes during nonconservative motion and double cross slip of dislocations was estimated. Their effect on the macroscopic behavior of samples is shown.     

Structure,unelastic properties,and deformation behavior of ultrafine-grained titanium

The results of investigations into the structure, unelastic properties, deformation behavior, strength, and plasticity of ultrafine-grained titanium produced by equichannel angular pressing are discussed. Particlular emphasis has been placed on the grain-boundary unelasticity and the effects of external thermal and thermal-force actions on the deformation behavior and plastic deformation localization at the meso- and macroscale levels. The influence of cold plastic deformation of ultrafine-grained titanium on the grain-boundary unelasticity and temperature dependence of the mechanical properties is considered.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 33–43, September, 2004.     

Lattice curvature evolution in metal materials on meso- and nanostructural scales of plastic deformation

The paper generalizes results of electron microscopy studies of structural states with high lattice curvature which arise in a wide class of materials under various conditions of severe plastic deformation: rolling, equal channel angular pressing, mechanical activation in planetary ball mills, and torsion in Bridgman anvils. The states are divided into two types: 1) a substructural state with elastoplastic lattice curvature of tens of degrees per micron due to high density of like-sign excess dislocations; and 2) a state with elastic lattice curvature up to several hundreds of degrees per micron in volumes of several nanometers. Analysis is performed to inquire into the formation of these states, peculiarities of their evolution, and their role in different mechanisms of plastic deformation and formation of nanocrystalline structures.     

Three-dimensional quantification of texture heterogeneity in single-crystal aluminium subjected to equal channel angular pressing

Abstract

A three-dimensional crystal plasticity finite element method (3D CPFEM) modelling of a real equal channel angular pressing (ECAP) process for investigating the mechanical properties and texture evolutions of single-crystal aluminium has been developed for the first time. The challenge of modelling such a severe plastic deformation via 3D CPFEM is how to accurately predict the deformation mechanism under the complicated contact conditions between a billet and a die. The validation by comparison with experimental observations demonstrates that the developed 3D CPFEM ECAP model is able to precisely capture the deformation characteristics at the microscale. Furthermore, this research clarified the previously remaining disputes such as the microstructural formation mechanism in the deformed area and the deformation nature in the plastic deformation zone. It is also the first time to extensively discuss the orientation-dependent deformation feature of the ECAP-processed billets, including morphology, lattice rotation angle and grain refinement.     

Grain boundary structure of nanocrystalline Cu processed by cryomilling

The microstructures of cryogenically ball-milled Cu were investigated by high-resolution electron microscopy. It was found that the grain-size reduction is a dislocation-controlled continuous process which consists of the formation of small-angle grain boundaries (GBs), a gradual increase in misorientations as a result of accumulation of more dislocations and, finally, the formation of large-angle GBs. The GBs were generally curved, wavy or faceted, and heavily strained, which are typical characteristics of nanostructured materials. In addition, extrinsic dislocations were found in many GBs, indicating that most are in a high-energy non-equilibrium configuration, which is consistent with observations in equal-channel angular pressing processed Cu, Ni, and Al-Mg, repetitive corrugation and straightening processed Cu and room-temperature ball-milled Cu. These results support a still-disputed concept that GBs in nanostructured metals processed by severe plastic deformation are mostly in non-equilibrium states.     

Dynamic pressing of titanium for producing an ultrafine-grained structure

Dynamic equal-channel angular pressing, a new method for the intense plastic deformation of materials, was developed and applied to titanium, as an example. A sample, accelerated in a gun to a velocity of 300 m/s, impinged on a matrix with intesecting channels. The deformation of titanium occurred at a shear-deformation rate of 10⁴–10⁵ s^?1 and pressure of several GPa. Upon deformation, the strength of titanium increased by a factor of 2, with the plasticity remaining at an acceptable level. Metallographic and electron microscopy analyses demonstrated that, under the action of intense deformation, the initial course-grained structure of titanium transforms into an ultrafine-grained one.     

Structure and phase composition of an Al-Mg-Li-Zr alloy under high-rate superplasticity conditions

Uniaxial-tensile tests are performed on samples of a commercial aluminum-lithium alloy subjected to equal-channel angular extrusion. It is found that the material under study has a highly fine-grain structure and exhibits superplasticity under tension. The microstructure of the samples is studied during their plastic deformation.     

Elasticity and anelasticity of microcrystalline aluminum samples having various deformation and thermal histories

The effect of the amplitude of vibrational deformation on the elastic modulus and internal friction of microcrystalline aluminum samples produced by equal-channel angular pressing was studied. The samples have various deformation and thermal histories. The elastic and inelastic (microplastic) properties of the samples are investigated. As the degree of plastic deformation increases, the Young’s modulus E, the amplitude-independent decrement δ_i, and the microplastic flow stress σ increase. As the annealing temperature increases, the quantities δ_i and σ decrease noticeably and the modulus E exhibits a more complex behavior. The experimental data are discussed under the assumption that the dislocation mobility depends on both the spectrum of point defects and the internal stresses, whose level is determined by the degree of plastic deformation and the temperature of subsequent annealing. The concept of internal stresses is also used to analyze the data on the effect of the degree of deformation and annealing on the rupture strength of the samples.     

Severe plastic deformation of amorphous alloys: I. Structure and mechanical properties

The basic regularities of variation in the structure and mechanical properties of amorphous Ni₄₄Fe₂₉Co₁₅Si₂B₁₀ alloy at severe plastic deformation (SPD) in a Bridgman cell at different temperatures are considered. It is shown that SPD is accompanied by homogeneous nanocrystallization, which is caused by the plastic flow mode. The transition from inhomogeneous mode of plastic flow to a qualitatively different one has been detected. The SPD structural model of deformational “dissolving” of crystals is proposed to explain why nanocrystals no more than 10 nm in size are observed during SPD. It is found that thermally activated nanocrystallization may occur at very low temperatures (77 K) under very high stress and with a high concentration of excess free volume.     

Modification of polyamide-6 structure by combined methods of solid-phase extrusion

Potentialities of new combined deformation schemes, including the solid-state extrusion through conical die (ED) and equal-channel multiple angle extrusion (ECMAE) implemented in different sequences to modify the structure and properties of semicrystalline polymers, have been studied for polyamide-6 (PA-6) as an example. It is shown that deformation by the ED–ECMAE scheme gives the best complex of physical and mechanical properties. A significal improvement of elastic and strength properties of PA-6 with a conserved high level of plastic characteristics has been observed.     

Influence of the deformation type and medium on the mechanodynamic penetration of nitrogen molecules into surface layers of armco iron

The extraction of nitrogen molecules from deformed samples of armco iron with different initial structures (annealed and subjected to equal-channel angular pressing) and different deformation prehistories (deformation in liquid nitrogen at 77 K, rolling in air at room temperature, and their combination) has been studied. It has been shown that the preliminary deformation in liquid nitrogen increases its concentration in the surface layer of the material and shifts the principal peak of its release toward low temperatures during heating. The results are associated with the existence of different types of nitrogen traps in annealed and nanostructured armco iron and with their changes during subsequent deformation.     

Effect of dislocation hardening on monotonic and cyclic strength of severely deformed copper

The present study aims at clarifying the role of dislocation strengthening in fatigue of materials manufactured by severe plastic deformation (SPD) techniques. Employment of single crystals hardened via equal channel angular pressing (ECAP) helps to minimise or completely eliminate the effect of high angle boundaries on strengthening and fatigue behaviour. Both monotonic strength and high cycle fatigue (HCF) resistance were improved significantly after the first ECAP pressing, when low-angle dislocation configurations dominate in the microstructure. The essential role of dislocation accumulation during severe plastic deformation is highlighted for both tensile and fatigue strength (SPD). Dilute alloying of copper by silver stabilises the deformation microstructure and further improves the fatigue properties considerably.     

Effect of preliminary strain hardening on the flow stress of titanium and a titanium alloy during shock compression

The effect of preliminary strain hardening of VT1-0 titanium and a Ti-6 wt % Al-4 wt % V alloy on their mechanical properties under quasi-static and high-rate (τ;10⁵ s^?1) loading is studied. Preliminary hardening is accomplished using equal-channel angular pressing (which results in a significant decrease in the grain size and a twofold increase in the quasi-static yield strength) and shock waves. High-rate deformation is attained via shock-wave loading of samples. The experimental results show that structural defects weaken the dependence of the yield strength on the strain rate. The difference in the rate dependences can be so high that the effect of these defects on the flow stress can change sign when going from quasi-static to high-rate loading.     

Recovery of Young’s modulus upon annealing of nanostructured niobium produced through severe plastic deformation

The effect of temperature (in the range 20–500°C) on the Young’s modulus of nanostructured niobium with Ta impurity content <0.5 wt % and that of O₂ < 0.1 wt % and with a mean grain size of ?200 nm is studied. The transformation of polycrystalline niobium into a nanostructured state is performed through severe plastic deformation by equal-channel angular pressing. The Young’s modulus is found to increase in two stages as the temperature of isothermal annealing is gradually increased. The mechanisms of recovery of the elastic modulus upon annealing of the nanostructured niobium are discussed in the context of the modern concepts of the defect structure of deformed metals.     

Structure and mechanical and electrochemical properties of ultrafine-grained Ti

A study is conducted into the microstructure and physico-mechanical properties of ultrafine-grained titanium produced by severe plastic deformation using the method of equichannel angular pressing. The effects of thermal and mechanical treatment on these characteristics are investigated. The possibility of forming mechanical properties in titanium that compare well with those of highly doped titanium alloys is shown. Institute for Strength Physics and Materials Science; Siberian Physical-Technical Institute at Tomsk State University; Institute of Physics of Advanced Materials at UFA Aviation Technology University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 77–85, January, 2000.     

Effects of severe-strain-induced defects on the mechanical response of two kinds of high-angle grain boundaries

ABSTRACT

Grain boundaries of metallic materials subjected to severe plastic deformation exhibit significantly enhanced diffusivity and excess energy compared with their relaxed poly- or bi-crystalline counterparts even when the macroscopic degrees of freedom are the same in both types of grain boundaries. Boundaries of excess energy are/can be relaxed by annealing. As a first step in accounting for this experimentally observed high-energy state of general high-angle grain boundaries subjected to severe plastic deformation, a concept of localised basic shear units and the presence of localised extra free volume in these units situated in different locations in the grain boundaries, which was originally proposed to explain steady-state structural superplastic flow, is made use of. Using MD simulation, the mechanical response of these modified grain boundaries is compared with that of their relaxed state. The results are also compared with a case of a homogeneous distribution of extra free volume within the grain boundary. The localised shear units containing extra free volume introduced in the grain boundaries are found to alter their physical and mechanical features strongly, which, in turn, drastically affect, consistent with experimental results, the mechanical response of the heavily deformed material.     

TEM studies of structure in OFHC copper processed by equal channel angular rolling

In this study, UFG (ultrafine-grained) structure formed through ECAR (equal-channel angular rolling) process has been studied by methods of electron microscopy. The microstructure, mechanical properties and microhardness were investigated in OFHC (oxygen free high conductivity) copper after 1st–13th passes. The interpretation of microstructure changes was performed using a model which describes mechanism of UFG structures formation. It was found that ECAR is a tool used for the purpose of achieving significant structural refinement resulting in a final grain size d ～ 400 nm. Moreover, this method provides such specific structural changes which have highly advantageous influence on mechanical properties (yield stress, ultimate tensile strength, reduction of area) as well as microhardness.     

Jumplike microdeformation of nanostructured metals

The parameters of microdeformation jumps for copper, aluminum, titanium, and Armco iron with the initial (annealed) structure and after equal-channel angular pressing are investigated in a creep mode under low compressive stresses. The strain rate is measured with a laser interferometer in 0.15-μm linear displacements. It is demonstrated that the values of the microstrain rate and the mean sizes of jumps for the annealed metals are larger than those for the metals subjected to severe deformation. It is revealed that there is a correlation between the jumps of microplastic deformation and the size of nanometal grains. The inference is made that, for nanostructured metals, as for other materials, the structural heterogeneity is one of the factors responsible for the jumplike deformation.